(1) Field of the Invention
The present invention relates to a process for the treatment of a lignocellulosic biomass with concentrated ammonium hydroxide and preferably with ammonia gas to increase the availability of structural carbohydrates (polysaccharides). Preferably, steam under pressure is used to strip ammonia from the biomass for recycling. In particular, the present invention relates to a process which enables the efficient conversion of the polysaccharides to monosaccharides preferably by enzymatic hydrolysis.
(2) Description of the Related Art
A wide variety of methods (e.g. concentrated or dilute acids or bases, high temperatures, radiation of various forms) have been used to pretreat lignocellulosic biomass to extract structural carbohydrates to be used to obtain monosaccharides for many different uses. The goal of these pretreatments is to increase the rate and/or yield at which the monosaccharides are subsequently obtained from the structural carbohydrates by chemical or biochemical means such as acid catalysis, enzymatic catalysis, fermentation or animal digestion. In general, these pretreatments have fallen short of desired economic and technical performance for several reasons: 1) many pretreatments degrade some of the sugars, e.g. to acids or aldehydes, thus reducing yields and inhibiting subsequent biological conversion of the remaining sugars; 2) when chemicals are used in the pretreatment, it is frequently difficult to recover these chemicals at reasonable cost; 3) residual chemicals can negatively affect downstream conversion operations; and 4) the effectiveness of many pretreatments is limited so that the ultimate conversions of structural carbohydrates obtained, independent of lost yield by sugar degradation reactions, is inadequate for competitive process economics. Thus there are many prior art methods, and they have numerous drawbacks including those outlined above.
Sufficiently inexpensive monosaccharides from renewable plant biomass can become the basis of chemical and fuels industries, replacing or substituting for petroleum and other fossil feedstocks. Effective, economical pretreatments are required to make these monosaccharides available at high yield and acceptable cost.
The prior art in the pretreatment of plant biomass with anhydrous liquid ammonia or ammonium hydroxide solutions is extensive. Illustrative are the following patents and literature references:    U.S. Pat. No. 4,600,590 to Dale    U.S. Pat. No. 4,644,060 to Chou    U.S. Pat. No. 5,037,663 to Dale    U.S. Pat. No. 5,171,592 to Holtzapple et al.    U.S. Pat. No. 5,865,898 to Holtzapple et al.    U.S. Pat. No. 5,939,544 to Karsents et al.    U.S. Pat. No. 5,473,061 to Bredereck et al.    U.S. Pat. No. 6,416,621 to Karstens    U.S. Pat. No. 6,106,888 to Dale et al.    U.S. Pat. No. 6,176,176 to Dale et al.    Felix, A., et al., Anim. Prod. 51 47-61 (1990)    Waiss, A. C., Jr., et al., Journal of Animal Science 35 No. 1, 109-112 (1972). All of these patents and publications are incorporated herein in their entireties.
In particular, ammonia fiber explosion (AFEX) represents a unique and effective pretreatment for biologically converting lignocellulosic biomass to ethanol (Dale, B. E., 1986. U.S. Pat. No. 5,037,663; Dale, B. E., 1991. U.S. Pat. No. 4,600,590; Alizadeh, H., F. Teymouri, T. I. Gilbert, B. E. Dale, 2005. Pretreatment of Switchgrass by Ammonia Fiber Explosion. Applied Biochemistry and Biotechnology, 121-124:1133-1141; Dale, B. E., 1991. U.S. Pat. No. 4,600,590; Dale, B. E., 1986. U.S. Pat. No. 5,037,663). In AFEX pretreatment, lignocellulosic biomass is exposed to concentrated ammonia at elevated pressures sufficient to maintain ammonia in liquid phase and moderate temperatures (e.g. around 100° C.). Residence times in the AFEX reactor are generally less than 30 minutes. To terminate the AFEX reaction, the pretreated biomass is depressurized (flashed). The AFEX process is not limited to anhydrous ammonia with AFEX. Some water is added to the biomass, so that any anhydrous ammonia is immediately converted into a concentrated ammonia water mixture on beginning the AFEX treatment.
Recovery of ammonia used in AFEX pretreatment is a key objective when integrating AFEX into a broader biomass conversion process design. The existing ammonia recovery design (Eggeman, T. 2001. Ammonia Fiber Explosion Pretreatment for Bioethanol Production, National Renewable Energy Laboratory (NREL) Subcontract No. LCO-1-31055-01), which is depicted in FIG. 1, calls for compressing ammonia, which is vaporized as a result of the flash operation, and separating liquid ammonia that remains in contact with the pretreated solids via evaporation in a dryer. The resulting vapor, which also contains water, is then delivered to a distillation column to purify the ammonia. The ammonia from the column is pumped up to pressure and, together with the compressed flash ammonia, is recycled to the AFEX reactor. FIG. 1 shows the existing ammonia recovery approach.
FIG. 1 shows the prior art system 10 including a closed AFEX reactor vessel 12 into which biomass, water and ammonia are introduced under pressure. Valve V1 is used to release pressure from the vessel 12. The treated biomass is transferred to a heated dryer 14. The dried biomass is transferred out of the dryer 14 for subsequent treatment. Ammonia from the dryer 14 is condensed by condenser 22 and sent to slurry column 16. Water is removed and condensed by condenser 18. Ammonia is condensed in condenser 20 and recycled to the vessel 12. Ammonia gas is pressurized in a compressor 24, condensed and recycled into vessel 12.
The problem is that the processes either produce low yields of the monosaccharides and/or require large amounts of liquid ammonia or ammonium hydroxide solutions.